I. Field of the Invention
This invention relates to imaging systems, and more particularly, to an imaging system employing liquid ink development.
II. Description of Prior Art
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface, e.g., as by heat or pressure. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image directly by charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired.
Similar methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. Included within this group are the "cascade" development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the "powder cloud" technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the "magnetic brush" process disclosed, for example, in U.S. Pat. No. 2,874,063.
Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of an electrostatic latent image may, for example, be obtained by flowing the developer over the image bearing surface, by immersing the imaging surface in a pool of the developer or by presenting the liquid developer on a smooth surfaced roller and moving the roller against the imaging surface.
A system is provided wherein the principles as disclosed by Walkup in U.S. Pat. No. 2,825,814 are applied in the process of the present invention. In the Walkup patent means and apparatus are disclosed for the formation of an electrostatic charge pattern or xerographic electrostatic latent image. An electric field is imposed through a photoconductive layer and to a contiguous insulating receiver while the photoconductive layer is subject to the action of a pattern of light and shadow of visible light or other activating radiation. The contiguous insulating surface is positioned adjacent the surface of the photoconductive layer and is spaced therefrom by an extremely minute distance such as, for example, the small gas or air gap existent in a condition of virtual contact of one surface with another. As stated by Walkup it is believed that electric charge under the influence of an applied field through the photoconductive layer and through the insulating layer migrates through the photoconductive layer preferentially at those areas exposed to activating radiation, causing deposition of charge on the insulating layer because of electric breakdown occurring in the gas gap which may exist between this insulating layer and the photoconductive surface, again under the influence of the applied field. The insulating receiver which may be an image receiving member, for example paper, may then be developed at a remote location.
Though imaging systems of the type described have been used to reproduce selected images, there is a constant need to impart to these systems a higher degree of sensitivity so as to provide higher quality images.